So Northrop Grumman -- maker of a system to protect planes from shoulder-fired missiles -- is trying to tap a market that bridges the military and civilian worlds.

The defense contractor Wednesday urged the Defense Department to install its anti-missile system on commercial airlines that transport soldiers and military equipment to war zones.

"We believe that the most immediate and best use of this kind of technology is to protect those aircraft, those commercial civilian airplanes that you and I fly on, that are called into service to support military troop and cargo movements in and out of those areas today," said Jack Pledger, a Northrop Grumman executive.

The Civil Reserve Air Fleet -- civilian airlines that transport troops under contract with the Pentagon -- could be a toe in the door for Northrop and BAE Systems, both of which have spent millions developing anti-missile systems but have found the airlines to be reluctant customers.

Airlines, defense contractors and terror experts disagree about the threat from shoulder-fired weapons, also known as MANPADS, for man-portable air defense systems.

Pledger said Wednesday that 27 known terrorist groups are believed to possess shoulder-fired weapons, that aircraft are vulnerable to the missiles within 25 miles of airports, and that one missile incident could have catastrophic effects on the U.S. economy.

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But other experts say the threat is exaggerated, and that most black market weapons are old, in disrepair, and incapable of bringing down large aircraft. They say the threat does not justify the cost, which could be up to $1 million per aircraft.

The anti-missile systems work by detecting incoming missiles and emitting an eye-safe laser beam that interferes with the missiles' guidance system and deflects the missile away from the plane.

Airlines are concerned that they may have to bear some of the cost, that maintenance of the devices would not comport with existing maintenance schedules, and that false alarms would throw their operations into disarray.

The Department of Homeland Security has been working with defense contractors to adapt military anti-missile systems for use on civilian aircraft. It has overseen initial development of the systems created by Northrop Grumman and BAE Systems.

The Northrop system Wednesday concluded a 14-month test during which anti-missile systems were installed on 11 FedEx cargo planes that flew 4,500 flights totaling over 10,000 hours.

Pledger said Northrop's Guardian System, which weighs 500 pounds, increased fuel consumption by less than three-quarters of one percent, an amount he called "negligible."

This summer, BAE Systems plans to install its units on three American Airlines planes that will be tested as they make cross-country flights for the remainder of the year.

One airline group Wednesday endorsed the idea of placing the anti-missile systems on troop transports.

Thomas Zoeller of the National Air Carrier Association, which represents 14 small air carriers, said the systems would improve safety and open up airports that are now off limits to his member airlines because of the threat of missile attacks.

If anti-missile systems were installed on aircraft, he said, his airlines could contract with the Pentagon to fly soldiers and equipment to sites closer to danger spots.E-mail to a friend